Integrated circuits are manufactured using photolithographic processes that direct light through a transparent photomask, also known as a reticle, to project a circuit image onto a silicon wafer. However, the process to make the photomask often results in the formation of defect seeds in the finished photomask which can lead to delayed and amplified interference during the patterning process. These defect seeds can grow with time in a fab environment due to the presence of airborne impurities and its potential reaction with the photomask surface, in the presence of activating mechanisms such as ultraviolet (UV) radiation. If the physical size of the defect exceeds the critical resolution, then the defect will cause undesirable image (pattern) transfer to the wafer. The quality of the integrated circuits thus produced may be adversely affected by any seed defects present in the mask. Because seed defects are not uncommon in the masks, these defects must be reduced or eliminated before using the mask in a production process.
The conventional method of cleaning a photomask has employed a cleaning method based on RCA cleaning (cleaning with a mixture of an acid such as sulfuric acid and hydrogen peroxide), which is also widely used for cleaning wafers. Typically, in a first step, cleaning is effected with a hot mixture of sulfuric acid and hydrogen peroxide to decompose organic objects such as resist and residual solvent present on the surface of a photomask and remove metallic impurities. This step provides the surface of the photomask with an improved wettability that enhances the efficiency of the subsequent cleaning steps. See U.S. Pat. No. 6,494,966 assigned to International Business Machines Corporation, which describes the use of acidic wash/rinse solutions to clean photomasks.
In a second step, the photomask is rinsed with hot pure water to displace the sulfuric acid cleaning agent from the surface thereof. Next, in a third step, the photomask is dipped in and cleaned with aqueous ammonia for the purpose of neutralizing residual sulfuric acid and removing attached foreign objects. During this step, ultrasonic waves can be applied to the dipping tank to facilitate the removal of foreign objects. During the cleaning, rinsing and drying steps, it is often preferred to contact the photomask surface with electrolytic solutions, such as those comprised of weak or strong acids or bases, such as aqueous sulfuric acid, aqueous ammonia or carbonated water, in order to avoid electrostatic discharge (ESD) due to static charge build-up, which can arise from shear liquid flow over the substrate surface when using highly resistive rinse fluids such as deionized water, as described in U.S. Pat. No. 4,569,695. It is known to those skilled in the art that ESD can create melt, distort, bridge or cut the reflective patterns created on the photomask substrate, therefore degrading the quality of the finished photomask (see U.S. Pat. No. 6,596,552).
Alternatively, the third step can involve cleaning with pure deionized water or mixed with a detergent, and optionally in an ultrasonic bath. The photomask can be eventually rinsed with pure deionized water as a forth step and then dried in a fifth step. The combined use of ionizing bars over a photomask rinse tank with pure deionized water can also prevent the occurrence of ESC (A. Steinman, Semiconductor Manufacturing 21 (1994) 179). Yet another method to rinse photomasks consists on the use of electrolyzed water, also known as ‘anodic’ water or ‘cathodic’ water to those skilled in the art (see U.S. Pat. No. 6,277,205).
Aqueous sulfuric acid used in conjunction with hydrogen peroxide as a photomask cleaning agent leads to strong adsorption of sulfate ions to chrome oxide (Cr2O3), such as the Cr2O3 layer naturally present on the surface of the Chrome (Cr) structures that compose the pattern layout of a photomask. The adsorption of sulfate-containing salts to chrome oxide and its dependence on pH has been studied in detail (G. Horanyi et al., J. Solid State Electrochem. 6 (2002) 485). Similar adsorption behavior is expected from the contact of sulfuric acid cleaning solution with quartz substrates. Furthermore, the surface neutralization reaction between residual sulfuric acid and aqueous ammonia results in potential incomplete displacement of sulfate ions from the reticle surface due to the adsorption process. Instead, formation of ammonium sulfate crystal seeds that remain present on the chrome or quartz surface during actual use and storage of the reticle in a fab environment is verified (K. Bhattacharyya et al., Proc. SPIE 4889 (2002) 478). During the lifetime of a reticle in a semiconductor manufacturing environment the mask surface is exposed to a variety of airborne contaminants, originating from the mask making materials, reticle containers and fab or scanner environment, such as outgassed products from the pellicle adhesive, gaseous ammonia and nitrous oxide generated during semiconductor fabrication processes and environmental carbon dioxide (E. Johnstone et al., Solid State Technology, May 2004). The reaction of such airborne contaminants with the ammonium sulfate defect seeds originated from the mask manufacturing process and present on the reticle surface result in the growth of such defects over time. This growth can even be assisted and accelerated by UV light, such as 193 nm or 248 nm imaging radiation used in photolithographic processes (J. Flicstein et al. Applied Surface Science 138 (1999) 394). The critical size beyond which a defect grown on the reticle surface becomes a printable killer defect on the wafer depends on the location and proximity of the defect with respect to intra-level and inter-level functional features within the chip layout.
To remove relatively large physical defects from a photomask, the industry often relies upon detection of the defect followed by laser ablation (see U.S. Pat. No. 6,593,040). Unfortunately, the size and shape of ammonium sulfate defect seeds created on the reticle surface is smaller than the resolution limit of the reticle inspection tools used to qualify the photomasks during the manufacturing process.
An alternative method that is commonly practiced to remove organic contaminants is described in U.S. Pat. No. 6,387,602. The procedure consists on using ultraviolet light in an oxygen environment (photo-oxidation) to remove trace organic objects that can remain on a surface of a photomask during fabrication and storage at the IC fabrication facility. This method results highly inefficient for the removal of ammonium sulfate defect seeds due to the inorganic nature of such impurities.